1. Field of the Invention
The invention pertains to the field of semiconductor devices. More particularly, the invention pertains to a wavelength tunable semiconductor vertical cavity surface emitting laser (VCSEL).
2. Description of Related Art
The wavelength tunable semiconductor laser plays an important role in wavelength division multiplexing transmission systems, wavelength division switching systems, wavelength cross-connection systems as well as in the field of optical measurements.
Prior art in this field includes far wavelength tunable edge-emitting lasers (M. Yamaguchi et al., U.S. Pat. No. 5,541,945) using different sections of the same waveguide structure of the device. Another way to achieve wavelength tunability for both edge- and surface emitting lasers is to use external mirrors or diffraction gratings, where the tuning is realized by mechanical tuning of the effective cavity length of the device (P. Tayebati, U.S. Pat. No. 5,949,801). In vertical cavity surface emitting lasers the tuning of the cavity length may be realized by using different micro-electromechanical systems. The disadvantage of these devices is the long tuning time related to the mechanical nature of the effects used. Frequency-modulated signal transmission systems are not possible using this approach.
Therefore, there is a need in the art for better wavelength control and tuning.
A wavelength tunable semiconductor vertical cavity surface emitting laser which includes at least one active element including an active layer generating an optical gain by injection of a current, and at least one phase control element, and mirrors that can be realized, e.g., by distributed Bragg reflectors. The phase control element contains a modulator exhibiting a strong narrow optical absorption peak on a short wavelength side from the wavelength of the laser generation. The wavelength control is realized by using a position-dependent electro-optical effect. If a reverse bias is applied, the absorption maximum is shifted to longer wavelengths due to the Stark effect. If a forward bias is applied, a current is injected and results in the bleaching and reduction of the peak absorption. In both cases a strong modulation of the refractive index in the phase control element occurs. The effect tunes the wavelength of the cavity mode, and the sign and the value of the wavelength shift are defined by the position of the modulator. Two phase control cascades can be implemented into the laser, one of which shifts the wavelength of the emitted light to larger values, and the other shifts the wavelength of the emitted light to smaller values. A power equalizing element can be used in such laser allowing either to maintain the constant output power at different emission wavelengths or to realize an independent frequency and amplitude modulation. A photodetecting element can be implemented in the laser allowing calibration of the laser for all operations.